In the horror genre, The Wicker Man and it's not even close.
Most scary films aren't in the horror genre. It's all the really weird stuff that's actually scary rather than the stuff that's meant to be scary... Like that short film in black and white where God is disemboweling himself... Literally what the **** is that. You can become desensitized to "scary" but not to "weird", weird always gives you an uneasy feeling and can haunt you.
One of the scenes in "Irreversible" traumatized me. I started getting panic attacks with loud repeating bass music in clubs after that scene with the fire extinguisher - the thudding bass reminded my brain of the fire extinguisher slamming the guy's head and I imagine it as me. I saw it out of context so I interpreted it that they broke the guys arm then raped him while caving his head in with a fire extinguisher, which is a lot worse than what actually happens in that scene.